This invention relates to a shut-off seal for a rotatable shaft which enters a fluid holding tank through an opening below the fluid surface. The invention has particular application to large mixing tanks and the like equipped with an agitator driven by a rotatable shaft. The drive means for the shaft ordinarily is located outside the tank at a relatively low level, so that arrangements must be made for sealing the tank at the point of shaft entry. Rotary seals for such purposes are well-known, but they are subject to wear and must be replaced from time-to-time. In order to service the seal it is necessary either to empty the tank or to provide a shut-off seal for preventing fluid leakage during the servicing operation.
Larkins et al., U.S. Pat. No. 4,878,677, discloses a shut-off seal comprising a moveable housing which receives a stuffing box. The stuffing box seals the shaft against liquid leakage during normal operational use. When the stuffing box seals become warn, the housing is moved into engagement with a collar which is secured to the shaft adjacent the inside surface of the tank. A shut-off seal is created between the surface of the collar and a surface of the housing. This necessarily requires transverse movement of the stuffing box along the axial direction of the shaft. Once a shut-off seal has been achieved, then the stuffing box may be serviced as required.
In many applications it is desirable to employ a mechanical seal for sealing of the shaft during normal operation. Such seals are well-known and are able to seal a rotating shaft quite effectively. Unfortunately, conventional mechanical seals cannot be moved transversely along the shaft to accommodate shut-off sealing as taught by U.S. Patent 4,878,677. A need therefore exists for a shut-off seal which may be operated from a point outside a fluid holding tank without moving the working rotary seal.